This project investigates the role of genetic factors in primate vocal development. Research strategies employed in these studies include analysis of vocal traits in hybrid offspring having vocally distinctive parents, comparing the vocal traits of full and half sibs with unrelated age-matched infants, and analysis of the effects of differential rearing on normal vocal development. This year, additional evidence was found in 2 hybrid squirrel monkeys studied through their first year of the different isolation call inheritance patterns between crosses involving vocally distinctive sympatric (Peruvian Gothic x Peruvian Roman) and allopatric (Guyanan Gothic x Bolivian Roman) populations. A pilot study of the effects of restricted rearing on vocal and other social behavior in squirrel monkeys suggests that exposure to conspecifics during the first year of life is not essential for normal expression of isolation and display calls, nor for participation in play and quiet affiliative interactions following introduction into a social group. An ongoing comparative study of prosimian isolation call characterstics obtained the first recordings from a 6-month old Lemur mongoz, and added data from a second infant Lemur fulvus sanfordi. Infant isolation calls from representative Lorisidae (lorises and galagos) were found to consist of single or multiple clicks, in striking contrast to the tonal isolation calls occurring in all other primates so far examined.